Wire-bonding Technology

(4) They should have certain electrochemical properties.

(5) They should be harmless to human, microsystems and the environment.

(6) They should be easy to remove.

Categories: Based on their melting points, filler metals can be divided into two cate-gories: brazing filler metals and soldering filler metals. The melting points of brazing filler metals are higher than 450^◦C, while the melting points of soldering filler metals are lower than 450^◦C. Correspondingly, welding can be categorized into brazing and soldering. Figure 4.2^[1]is a list of common filler metals.

The eutectic solder⁶³Sn/³⁷Pb, with a melting point of 183^◦C, has been the most widely used solder in electronic engineering interconnection for decades. In order to meet require-ments for environmental protection, researchers are developing and using lead-free solder series such as Sn-Ag, Sn-Zn, and Sn-Bi.

4.3 Wire-bonding Technology 4.3.1 Basic Concept^[2]

Wire-bonding is a method whereby a chip with active surface facing up is attached to a package or a substrate and then the I/O pads on the chip are connected with the I/O leads of the package or the wiring pads on the substrate with a fine wire such as gold or aluminum wire.

Wire-bonding plays a very important role in interconnection technology in electronic engi-neering for its maturity, simple technique, low cost, and strong applicability, and it is widely used in microsystems packaging. It is still in change and developing to cater to and satisfy the demands of new techniques and materials of semiconductors.

For over ten years people have been predicting that it will be outdated in the near fu-ture. This technology, however, didn’t disappear; it is still actively used from low-end to high-end packages as one of the mainstream interconnection technologies, and continues its development along with the development of microelectronics systems technology.

4.3.2 Types of Wire-bonding^[3,4]

Depending on the degree of bonding automation, wire bonding can be divided into manual bonding, semi/automatic bonding, and auto bonding. Based on the bonding process, it can also be divided into three types: ultrasonic bonding, thermocompression bonding, and thermosonic bonding. These three processes have their own features and are applicable for different products.

Aluminum wire bonding is usually performed in an ultrasonic bonding process at room temperature. In this process, the high-frequency vibration energy generated from an ultra-sonic generator is transferred to the contact surfaces of the aluminum wire and bonding pad through the bonding head under a compression force. A firm bond is formed by the “atomic bonding” between the two intimate contact surfaces. The bonding head is typically a wedge type. Therefore, this kind of bonding is also called wedge bonding.

Thermocompression bonding involves the use of temperature and pressure to produce plastic deformation of metal on the bonding pad, as well as destroying the oxide layer on the interface of the metal bonding pad so as to make the interface of both bonded metal wire and bonded pad metal reach atomic attraction bond and realize bonding through the attraction force between atoms. In addition, the rough metal interface together with the use of temperature and pressure can help close embedding between the metals. This bonding process, however, may cause damage to metal wire from over-distortion, as well as affecting bond quality. This limitation restrains the use of thermocompression bonding.

68 Chapter 4 Interconnection Technology Thermosonic bonding is also called gold ball bonding. The principle of thermosonic bond-ing is similar to that of thermocompression bondbond-ing, except that themosonic bondbond-ing also applies ultrasonic energy. The process of thermosonic bonding is shown in Figure 4.3:

(1) melt one end of metal wire into a ball through high-voltage electric flame off; (2) apply temperature and ultrasonic pressure energy on the bonding pad of the chip to produce inter-face plastic deformation and destroy the oxide film of the interinter-faces for activation; (3) make diffusion between the two interfaces to be bonded through contacting two metals to form the first bonding spot; (4) move the bonding head to the bonding point of the leadframe or the bonding pad of the substrate through precise and complicated dimensional control;

(5) apply temperature and ultrasonic pressure for the bonding of second bonding spot; (6) raise the controlled tail length of the bonding wire for the next ball formation; (7) repeat (1–6) steps and finish the connection of the second wire, the third wire, etc. The obvious difference between the two bonding joints is, during the processing of the first bonding joint, the end of the metal wire should be melted to a ball shape, while in the processing of the second bonding joint, it is not necessary to melt the end of the metal wire into a ball shape;

the pressure from the specific shape of the wedge snaps the metal wire instead of melting it.

Since thermosonic bonding occurs at a temperature lower than that for thermocompression and the bonding strength is enhanced, it has become the main bonding method in Au wire bonding. More than 90% of the bonders used in production lines today are automatic Au wire bonders adopting the thermosonic bonding technique.

Metal wire

Pressure head Electric spark

Ball bonding

Ultrasonic

Ultrasonic Pressure

Wire clip

Wedge bonding

(a) (b)

(c) (d)

(e) (f)

Figure 4.3 Illustration of thermosonic bonding process

4.3.3 Materials Used in Wire-bonding

Different bonding materials will be chosen by different bonding methods. A gold wire, for example, is mainly used for thermocompression bonding and thermosonic bonding; an alu-minum wire or alualu-minum alloy (Si-Al, Cu-Si-Al) wire is usually used for ultrasonic bonding.

Basic requirements of materials for wire bonding are as follows:

(1) They are capable of forming low-resistance ohm contact with bonding material, such as aluminum, gold, or other interface materials.

(2) They have high bonding strength with bonding materials.

(3) They have good conductivity.

4.3 Wire-bonding Technology 69 (4) They have good plasticity for bonding technique and good shape stability.

(5) They are chemically stable.

Tables 4.1 and 4.2 list the features of gold wire and aluminum wire, respectively.^[2,4]They show that both gold wire and aluminum wire have greatly improved ductibility and flexibil-ity after the annealing process and they become more suitable for defect-free bonding. Since there is a different melting point between aluminum and gold wires, the annealing temper-ature must be different to achieve the ideal effect. For example, the annealing tempertemper-ature for aluminum is relatively low, while that for gold is slightly higher.

Table 4.1 Properties of gold wire used for wire bonding

Diameter Weight Average minimal ductibility Failure Resistivity

(μm) (mg/m) Before annealing(%) After annealing(%) strength(N) (Ω/m)

127 244 5 15 1.6 1.84

Table 4.2 Properties of aluminum wire used for wire bonding

Diameter Average minimal ductibility Failure strength(N) Resistivity (μm) Before annealing(%) After annealing(%) Before annealing After annealing (Ω/m)

127 1.5 15 4 2 2.23

In addition to gold and aluminum wire, copper has also been used for IC interconnection in recent years. Since the gate delay of an integrated circuit depends on the resistance and capacitance of interconnection material, copper with a higher conductivity than that of conventional aluminum is good for reducing resistance and gate delay. The use of copper as an interconnection material, however, depends on improving the bonding technique. The protection layer of a copper bond pad, for example, is required to prevent the oxidization of copper. During wire bonding, it is necessary to use the multilevel drive of the ultrasonic energy converter, which can destroy the oxide layer on the surface of copper with high frequency ultrasoni cenergy and finish diffusion bonding with lower frequency ultrasonic energy.

In addition to its advantage of low resistance, copper wire has lower cost and higher strength and stiffness in comparison with gold, and it is suitable for fine-pitch wire bonding.

Copper also has low intermetal diffusivity, and its intermetallic compound grows slowly so that the electric resistance of the infiltrating layer between metal layers is low. For copper wire bonding, protective gas should be used in electronic ignition to prevent copper ball oxidation.

4.3.4 Key Process Parameters of Wire-bonding^[4,5]

(1) Temperature control: The temperature range is from room temperature to about 400^◦C. Temperature control accuracy is better than 1^◦C. Temperature can be controlled manually or automatically by controlling the current of the heater.

(2) Precisely positioning control: Precisely positioning control of the chip, lead frame, and package substrate is usually accomplished by a combination of precisely controlling the guide track, precision jigs, and vision system.

70 Chapter 4 Interconnection Technology (3) Setting of bonding parameters: parameters^[6], such as current, voltage, frequency, and amplitude for driving the ultrarsonic energy converter, wire clip, electronic flame, bonding pressure, and time are carefully determined to ensure the precision of bonding spot, bonding quality, and long-term reliability.

Figure 4.4 is a sketch map of the bond head of a bonder. The manufacturing of the ultrasonic energy converter, wire clip, and wedge requires advanced fabrication techniques and inspection/testing skills and thus is more valueable. The key technology involved in this part includes: program-controlled ultrasonic energy converting technique, gold ball forming technique, bonding pressure control, and wire clip precision movement control.

Figure 4.4 The structure of a bond head

The ultrasonic energy converting technique involves precision supporting wedge, the vi-bration energy, and pressure transferred to the wedge. The key point of the technology is the control of output frequency and voltage of the ultrasonic generator to regulate the resonance frequency and amplitude of the ultrasonic energy converter, which is very important to the quality and reliability of the bonding spot.

The way a gold ball is formed has a direct effect on the performance of the ball bonding joint. It controls the time and energy of the high voltage electric charge to melt the tip of the gold wire and form a small ball under the surface tension. In gold wire bonding, in order to control the bonding quality, consistent ball diameter is required. Thus, automated bonders are usually equipped with ball diameter detectors. Gold ball diameters are typically controlled within 1.4 to 3 times the diameter of gold wire.

Bonding pressure is precisely controlled by a DC motor driving through the feedback signals of a proximity sensor. The bonding force is typically controlled within 10–20 grams with a resolution of 0.002 Pa. The wire clip driver is driven by a piezoelectric ceramic actuator. Its performance in opening and closing is very important to ensure a rapid and accurate bonding process. When the wire clip is closed, enough holding force should be used to ensure the snap of wire without damaging the integity of wire. When the wire clip is opened, enough space should be provided to let the wire through without any obstacles.

Though it is easy to achieve single drive and control of every functional part of the bonding sets, it is not easy to fulfill these performances rapidly, precisely, orderly, and harmoniously, because they are an integrated unit assembled together with dependent and cooperative functions. It is a remarkable indication of qualified assembling testing, a symbol of reasonable electric design and software control, as well as the key point of reliability and joint bonding precision. Therefore it is necessary to draw a precise sequence diagraph showing the electronic flame, ultrasonic energy converter, clip drive, bonding pressure, and

4.4 Tape Automated Bonding Technology 71